This invention is generally related to purifying a gas, and more specifically, this invention relates to purifying hexafluoroethane gas which contains non-volatile residues when it is stored and distributed from certain steel or aluminum cylinders.
Hexafluoroethane, C2H6, is an etchant material used in the fabrication of semiconductor devices. If hexafluoroethane contains impurities including non-volatile residues (xe2x80x9cNVRxe2x80x9d) it will likely deposit in certain process regulators, filters, lines and chambers after the system, and the pressure may reduce to near atmospheric pressure. NVR, an undesired impurity, may be present in the form of heavy hydrocarbon or fluorocarbon oils or solids from the hexafluoroethane that is near its critical temperature (19.7xc2x0 C.) and critical pressure (29.8 bar or 432.1 pounds per square inch), This deposit may cause component failure or may contaminate the semiconductor devices during fabrication.
Generally, hexafluoroethane is packaged in both aluminum and steel cylinders, and the NVR impurities are different in each of these types of containers. It is believed that a major source of NVR contaminants is from lubricants used to insert the valve into the cylinder. It is also believed that the impurities in the aluminum cylinders are from mostly fluorocarbon oils or solids, whereas the impurities in the steel cylinders are from mostly hydrocarbon oils or solids.
If the NVR levels exceed a concentration of approximately 50 parts per billion (by weight), it must be purified to a concentration below that threshold before it can be used by a customer.
Various materials presently available for removing impurities at the specific temperature and pressure of interest, including coalescing oil filters, charcoal beds, 3A, 4A, 5A, 13X molecular sieve beds, alumina beds, silica gel beds, glass wool filters, borosilicate glass filters and fiberglass filters were made, but were unsuccessful in removing impurities. None of these approaches remove NVR by themselves at the temperature and pressure of interest. In general the gas will actually contain more NVR after the use of these materials. This is because hexafluoroethane acts as a solvent and dissolves materials off the bed""s surface.
Pressure reduction, filtration and recompression as well as distillation will remove NVR. However, these approaches are not at the temperature and pressure of interest and are very expensive to perform, especially for a small number of cylinders.
There is believed to be no prior art that teaches or suggests the removal of NVR from hexafluoroethane from near its critical point.
U.S. Pat. No. 4,881,953 describes the use of a bed of activated carbon to remove heavy hydrocarbons from a gas stream. This approach did not work for hexafluoroethane.
J. A. Brink Jr. et al, in xe2x80x9cMist Removal from Compressed Gasesxe2x80x9d, Chem. Eng. Prog. 62(4), 60 (1966) describes the use of fiber elements in a coalescing style filter housing. The main discussion is on the removal of oil from compressed air at a pressure of up to 5,500 psig. This type of oil removal approach does not appear to be successful for hexafluoroethane. U.S. Pat. No. 3,997,303 discloses a liquid-gas phase separator having a perforated plate and mist eliminator pad. U.S. Pat. No. 3,802,160 discloses an aerosol coalescing filter. U.S. Pat. No. 5,800,597 discloses an integrated coalesce filter-membrane device to provide a filtered gas employing stream and system employing such a device.
U.S. Pat. No. 3,972,694 discloses the construction of a filter tube from a mass of interrelated non-woven glass fibers.
U.S. Pat. No. 4,826,497 discloses a fibrous absorbent which enhances deodorizing properties. This patent describes the use of a crystalline siliceous molecular sieve of which 90 percent of the framework is silicon dioxide tetrahedra for the removal of organic body odors.
U.S. Pat. No. 4,934,148 to Prasad et al. discloses a dry, high purity nitrogen production process and system in which multiple bed series are used to remove water from a gas system.
There is therefore a need to provide a system, and a method for using such a system, in which a series of adsorption beds are used in specific sequences and at specific temperatures, and which is economically advantageous. When the correct sequence and conditions are used, the NVR will be removed from the hexafluoroethane that is at the temperature and pressure of interest. Since this process works at the pressure of interest, this invention requires no recompression or need for distillation. Accordingly, installation of this invention requires less capital and the operation cost is lower when compared to the present methods for purification of hexafluoroethane using pressure reduction/recompression or distillation.
An aspect of this invention is directed to a method and a system for purifying hexafluoroethane near its critical temperature and pressure in which the hexafluoroethane contains at least 1 ppb by weight of a non-volatile residue. This method comprises passing the hexafluoroethane to a sand trap bed held at less than about 19.7xc2x0 C. to produce sand treated hexafluoroethane. Then the sand treated hexafluoroethane is passed to an adsorption bed assembly comprising a glass wool-molecular sieve-glass wool combination held at a temperature of greater than about 19.7xc2x0 C., and at a pressure of from about 100 psig to about 2,000 psig to produce sand and adsorption bed treated hexafluoroethane. As used herein, xe2x80x9cadsorption bed assemblyxe2x80x9d refers to a glass wool-molecular sieve-glass wool combination. The order for steps of passing through the sand trap bed and the adsorption bed assembly may be reversed. However, when the hexafluoroethane is passed through the sand trap bed as the last step, a filter is generally needed to remove undesired impurities.
In another embodiment, hexafluoroethane may be purified near its critical temperature and pressure by passing it through a sand trap bed alone, followed by filtering to remove undesired impurities.
Another aspect of this invention is a system for purifying hexafluoroethane near its critical temperature and pressure, wherein said hexafluoroethane contains at least 1 ppb by weight of a non-volatile residue. The system comprises a sand trap bed of fine granular consistency held at less than about 19.7xc2x0 C.; and an adsorption bed assembly made up of a glass wool-molecular sieve bed combination held at a temperature of greater than about 19.7xc2x0 C. in the following order comprising a first glass wool bed, a molecular sieve bed, and a second glass wool bed.
In still another embodiment, the system may comprise of a sand trap bed alone.
Various embodiments also provide for the use of either the sand trap bed or adsorption bed assembly individually.